The present invention relates to a magnetic field sensor using a highly sensitive MTJ (magnetic tunnel junction) device. Its applications include use as a current sensor capable of sensing very small changes in the current flowing through a conductor.
In a conventional current sensor, the sensing elements are two long GMR (Giant MagnetoResistance) bearing stripes disposed adjacent to a conductor to be measured in an in-plane direction[1,2]. Such a current sensor includes: the conductor bent into a U shape; first and second GMR elements located on the conductor on opposing legs of the U so that a magnetic field generated by a nearby conductor will induce opposite magneto-resistance changes in the two GMR devices; and first and second constant current sources that supply constant and equal currents to the first and second GMR elements. The current sensor further includes a differential amplifier that detects the difference between the voltage drops across the first and second GMR elements. The field current is then detected based on the difference between the two voltage drops.
The scheme described above is schematically illustrated in FIG. 1. As can be seen, the two GMR devices 11 and 12 both lie on the U-shaped current sensor, but on opposing legs thereof. Consequently, the reference layers of the two magnetic sensing devices 11 and 12 are pinned in opposite directions (relative to the magnetic fields generated by the current in each leg) thereby ensuring that their resistances will change by different amounts when exposed to a magnetic field generated by the same current I. An important feature of this structure is that it is insensitive to the influence of noise from undesired external small magnetic fields as well as to resistance variations due to temperature change.
If this scheme is to work properly, the magnetization of each GMR element's free layer has to remain in a single domain state. However, a large external magnetic field (one greater than about 60 Oe) could change the single domain state of a GMR or MTJ free layer into multiple domain states, leading to hysteresis of the magnetic response and causing current reading errors. To address this problem, the prior art device discussed above includes means for imposing on the free layer a longitudinal magnetic bias. Said means take the form of a pair of permanent or exchange coupled magnets, one such pair for each sensor. Aside from the additional cost that this adds to the manufacturing process, use of longitudinal bias will also reduce the sensitivity of the device which may be further degraded by changes in the permanent bias over time.
The present invention discloses a structure (and process for its manufacture) having a magnetic free layer that is domain stable without the need for longitudinal bias magnetization.
[1] US 2006/0071655
[2] US 2006/0170529
A routine search of the prior art was performed with the following references of interest being found:
U.S. Patent Applications 2006/0071655 and 2006/0170529 (Shoji) disclose first and second MTJ elements with resistance values opposite to each other and a differential amplifier to detect the difference between voltage drops. U.S. Pat. No. 6,930,479 (Xiao et al) shows an MTJ current sensor for a scanning magnetic microscope. U.S. Pat. No. 6,295,931 (Cutler et al) discloses MTJ magnetic field sensors.